A lithium ion secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte. The positive electrode includes a positive electrode current collector made of, e.g., aluminum and a positive electrode active material layer formed on a surface of the positive electrode current collector and including a positive electrode active material such as a lithium composite oxide. The negative electrode includes a negative electrode current collector made of, e.g., copper and a negative electrode active material layer formed on a surface of the negative electrode current collector and including a negative electrode active material such as a carbonaceous substance. An electrode group is formed by winding the positive electrode and the negative electrode with the separator interposed therebetween. The electrode group and the nonaqueous electrolyte are housed together in a battery case, thereby forming a lithium ion secondary battery.
Generally, when an internal short circuit occurs through the positive electrode active material layer and the negative electrode active material layer in a lithium ion secondary battery, a large current flows to the point of the short circuit and heat is increasingly generated in the battery, resulting in a risk of excessive heating of the battery. In order to avoid this phenomenon, lithium ion secondary batteries are equipped with a shutdown mechanism in which micro-pores formed in the separator are closed by the heat generated by the short circuit.
However, when external force crushes a battery (crushing), or an electric conductor such as a nail having a large diameter penetrates a battery (nail penetration), a rupture takes place in the separator, and an inner short circuit occurs in a larger area, thereby easily causing excessive heating. In that case, since heat is abruptly generated near the point of short circuit, a meltdown in which a portion of the separator surrounding the point of short circuit melts and shrinks takes place to cause further short circuits, resulting in a risk of excessive heating of the battery.
In order to solve the above described problem, it is proposed to form, in a battery, a heteropolar metal part facing zone by providing an exposed metal part which is at the same potential as the positive electrode and another exposed metal part which is at the same potential as the negative electrode such that the exposed metal parts face each other (see, e.g., Patent Document 1). In particular, with respect to a wound-type battery, it is proposed to form a heteropolar electrode current collector facing zone where a positive electrode current collector exposing section faces a negative electrode current collector exposing section. The positive electrode current collector exposing section is free of an active material layer and located in an outer circumferential portion of the positive electrode included in the electrode group. The negative electrode current collector exposing section is free of an active material layer and located in an outer circumferential portion of the negative electrode included in the electrode group. In addition, it is proposed to form a heteropolar metal facing zone which is formed by a current collector exposing section and a battery case being in electrical connection to the electrode having a polarity opposite to that of the current collector exposing section. These proposed configurations allow crushing or nail penetration to cause an inner short circuit with low short-circuit resistance, and accordingly, excessive heating can be prevented. Even if the crushing or the nail penetration progresses after occurrence of the short circuit and a point of short circuit is produced between the active material layers, since a short circuit occurring between metals having low specific resistance has short-circuit resistance lower than short-circuit resistance between the active material layers, a reduced current is allowed to pass between the active material layers, and accordingly, excessive heating of the battery can be prevented (see, e.g., Patent Documents 1 and 2).